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U5. Tidal Energy

  • Tides and waves -> mechanical energy
  • temperature difference (between surface and deep layers) -> Thermal Energy
  • Common disadvantages to above energy sources are
    1. Low energy density
    2. energy potential occurrence far away from consumption center
  • Tides are produced by gravitational attraction of moon and sun acting upon the rotating earth.
  • The moon exerts a larger gravitational force (about 70 per cent of the tide producing force) on the earth, as it is a great deal closer than the sun

  • The ocean level difference caused due to tides contains large amount of potential energy

    • Highest levels of tidal water -> Flood Tide or High tide
    • Lowest level of tidal water -> low tide or ebb
    • The difference between Flood Tide and ebb is known as Tidal Range

  • When the sun, earth and moon are aligned (approximately) in conjunction, the lunar and solar tides are in phase, producing net tides of maximum range

    • Spring Tides -> Occur Twice per lunar month at new moon and full moon
  • When sun-earth and moon-earth directions are perpendicular (in quadrature), the solar and lunar tides are out of phase producing net tides of minimum range.
    • Neap tides -> occur twice per month at times of half-moon

LIMITATIONS: 1. Extremely Site specific - due to Geographic and economical constraints economic recovery of energy from tides is feasible only at those sites where energy is concentrated in the form of tidal range of about 5 m or more and geography provide favorable site for economic construction of a tidal plant, thus it is site specific, 2. Tidal Power generation is not in phase with demand We demand 24hrs of energy and tidal energy is generated for 12hrs 35mins due to mismatch of lunar pull. 3. Changing tidal power ranges in 2 weeks periods causes change in power changeInTidalPower.png 4. Turbines required to operate at variable heads 5. Tidal plant disrupts marine life at location and harms the ecology 6. Location of sites may be distant from the demand centers

Ocean tides generates more energy that air for spinning turbines as ocean tides are 832 times denser.

Total Potential Energy in Water W:

\[ dW = dm.g.h ....(I) \]
\[ dm = \rho.A.dh ....(II) \]
\[ dW = \rho.A.g.h.dh\ .....(using\ II\ in\ I) \]
\[ W\ = \ \int\limits^{R}_{0} \rho.A.g.h.dh \]
\[ W = 1/2 \ * \ \rho * A * g * R^{2} \ \ \ Joules \]

where .. \(\rho\) -> density of water | g -> gravitational constant | R = tidal Range

Remember, Average sea water density = \(1025 \ kg/m^3\)

Tidal power is directly proportional to 1. Area of basin 2. Square of the tidal range

but rather than going till 0, the turbine should be stopped at minimum value 'r' below which the operation beocomes uneconomical.

Potential Energy in Water \(W = 1/2 \ * \ \rho * A * g * (R^{2} - r^2) \ \ \ Joules\)

\(Average\ power\ potential\ available = 0.225 * A * (R^{2} - r^2) \ \ \ Watts\)

Pasted image 20240421100057.png

Components: 1. Dam, Barrage or Dyke 2. Sluice Ways -> rapid controlled gates, used to fill basin during high tides or emptying it during low tides 3. Special, bulb type power turbine generator set steel shell containing an alternator and special Kaplan turbine with variable pitch blades.

2. Ocean Tidal Energy Conversion Schemes

    1. Single Basin Single Effect Scheme
    2. ebb generation cycle -> sluice way is opened to fill the basin during high tide
    3. Wait till suitable head is created
    4. allow water to flow through turbine coupled to generator till rising tide reduces the head to minimum operating point.
    5. flood generation cycle -> power generation during filling operation of the basin
    6. Sloping nature of the basin makes ebb generation to be more productive
    7. Increased output can be generated by pumping water during high tide to increase the basin level and therefore the generation head.
    8. Pasted image 20240421112831.png
    9. Pasted image 20240421112855.pngSingle basin single effect tidal energy conversion scheme
    1. Single Basin Double Effect Scheme
    2. Power generated at both flood and ebb
    3. Two way (reversible) hydraulic turbines are used
    4. Pasted image 20240421113233.png
    5. The routine is as follows:
      1. inward sluicing to fill the basin
      2. holding period
      3. ebb generation
      4. outward sluicing to empty the basin
      5. holding period
      6. flood generation
    1. Two basin linked Basin Scheme
    2. To maintain continuity of power supply
    3. one basin is filled up at high tide and other one is emptied at low tide
    4. permanent head is created ( A permanent gap between high tide and low tide) Pasted image 20240421115051.png
    1. Two Basin: Paired Basin Scheme
    2. two single basin schemes
      • One generates on flood cycle
      • One generates on ebb cycle
    1. Tidal Current Schemes
    2. The capital cost per unit of power produced appears to be high. As the tidal flow power lags about π/2 be/hindtidal range power from a single basin, the two systems could be complementary.

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